1. Field of Invention
The present invention relates to photolithography masks and more particularly to methods of reusing such masks in the manufacturing of integrated circuits.
2. Related Art
As is well known in the art, both precision and non-precision configuration masks can be used to customize integrated circuits by patterning specific images onto a photoresist layer of a semiconductor device during the photolithography processing step. A precision configuration mask is typically created by using precision mask-making tools, such as an electron-beam or pattern generator, to define all features on the mask. For non-precision masks, only the reticle alignment marks and other large peripheral features are typically created by using precision mask-making methods. Non-precision targeting energy beam tools, such as a non-precision laser (e.g., the Model 1225hp manufactured by Electro-Scientific Inc. of Portland, Oreg.), can then be used to define the desired non-precision configuration patterns. Note that for descriptive purposes only, any non-precision targeting energy beam tool will be referred to hereinafter as a non-precision laser.
Making both precision and non-precision masks requires precision mask-making tools because of the need to place the reticle alignment marks within the strict tolerances specified by stepper manufacturers. For example, ASM Lithography of Veldhoven, The Netherlands, requires that placement dimensions relative to the center of the mask be within .+-.0.25 microns for their Model 2500 stepper. Without a reference set of alignment targets already patterned on the mask, many non-precision lasers cannot meet these sort of tolerance requirements. If the alignment targets are not placed within the specified tolerance, the image on the mask may be shifted, resulting in an unacceptably large registration error of the pattern on the wafer.
Other features on the periphery of the mask do not require the placement accuracy and dimension control of a precision mask-making tool. Even so, non-precision targeting energy beam tools, such as lasers, are not a suitable alternative for creating the large geometries (e.g., 0.1 mm to 5 mm) typically found in this region of the mask due to the very small spot size (e.g. 3 .mu.m to 6 .mu.m) used by the laser. Therefore, using a non-precision laser to create these large features requires significantly more time than using a precision mask-making tool that uses either multiple beams, large area exposures, or higher raster rates for the large geometries. As a result, by using precision mask-making tools, very high write rates are possible when creating large peripheral features on the mask. Even though the cost of using a precision mask-making tool is higher, the overall write-time is significantly less, resulting in a lower overall cost than using a non-precision laser to create the large geometries.
However, because of the high cost of using precision mask-making equipment, it is desirable to further reduce the cost of producing the configuration mask by eliminating or reducing the use of the precision mask-making equipment.